1. Field of the Invention
The present invention relates to a technique for controlling the tension of a web of printing paper in a printing apparatus for printing on the printing paper while transporting the printing paper along a predetermined transport path.
2. Description of the Background Art
A web printing apparatus for printing on a web (or elongated piece) of printing paper while transporting the printing paper along a predetermined transport path has been conventionally known. The web printing apparatus performs printing in a predetermined printing position on the printing paper while transporting the printing paper by using a plurality of rollers including an infeed roller, an outfeed roller and the like.
Such a web printing apparatus applies a predetermined tension to the printing paper to ensure the flatness of the printing paper in the printing position. Conventional techniques have applied a predetermined tension to the printing paper by setting the rotation speeds of the respective rollers for transporting the printing paper at values increasing gradually in a downstream direction along the transport path. Such techniques, however, have required a user to perform the operation of setting the rotation speeds of the respective rollers. Additionally, the user has been required to set the rotation speeds of the respective rollers again each time the type of printing paper to be used is changed.
Japanese Patent Application Laid-Open No. 2002-248743 discloses a printing apparatus which measures the tension of printing paper to effect feedback control of the rotation speed of an infeed roller so that the result of measurement approaches a predetermined value. Such tension control eliminates the need for the user to set the rotation speeds of the rollers, and also achieves the automatic control of the tension of the printing paper regardless of the type of the printing paper used.
However, when the tension control disclosed in Japanese Patent Application Laid-Open No. 2002-248743 is effected in a plurality of locations in a web printing apparatus, there arises a so-called “hunting” problem. FIG. 5 is a graph showing variations in the tension of the printing paper in two (first and second) control positions in a web printing apparatus when the tension control disclosed in Japanese Patent Application Laid-Open No. 2002-248743 is effected in the two control positions. The abscissa of FIG. 5 represents time, and the ordinate thereof represents the tension value. The tension in the first control position (indicated by the solid curve of FIG. 5) and the tension in the second control position (indicated by the dash-and-dot curve of FIG. 5) are controlled to approach a predetermined value pv. However, because the web printing apparatus handles a single continuous piece of printing paper, the tension in the first control position and the tension in the second control position interact with each other. For this reason, the tension in the first control position and the tension in the second control position do not converge to the predetermined value pv but repeatedly swing on both sides of the predetermined value pv. Thus, it has been difficult for the tension control disclosed in Japanese Patent Application Laid-Open No. 2002-248743 to maintain the tension of the printing paper in the plurality of locations in the web printing apparatus at a constant value.